1. Field of the Invention
The present invention relates to electric power conversion devices for driving switching elements on the basis of an instruction current value and a currently-flowing current value.
2. Description of the Related Art
For example, Japanese patent laid open publication No. JP 2009-118571 discloses a direct current to direct current converter (DC-DC converter) as an electric power conversion device having a conventional structure for driving switching elements on the basis of an instruction current and a currently-flowing current.
The DC-DC converter disclosed in JP 2009-118571 adjusts a drive signal on the basis of a target current and an actual current which flows in a transformer and drives transistors on the basis of the drive signal. In general, the DC-DC converter is comprised of a switching circuit, a central processing unit (CPU) a digital to analogue conversion section (D/A conversion section), a current sensor and an absolute value circuit, a comparator, a hold circuit, an AND circuit and a drive circuit.
The switching circuit is comprised of four switching elements in which a pair of two switching elements is connected in series, another pair of other two switching elements is connected in series, and the pair of the two switching elements is connected in parallel to another pair of the two switching elements. The CPU generates a target current on the basis of a difference between a target voltage and an output voltage. The D/A conversion section converts the target current generated by the CPU to a voltage. The current sensor and the absolute value circuit detect an absolute value of the current which flows in the transformer, and output a voltage corresponding to the detected absolute value. The comparator compares the target current with the actual current which flows in the transformer. A pulse output section in the CPU generates and outputs a pulse signal having a predetermined time length and a predetermined duty ratio. The holding circuit and the AND circuit adjust the drive signal to be supplied to the transistors on the basis of the comparison result of the comparator. When an absolute value of the current flowing in the transformer is smaller than the target current, the holding circuit and the AND circuit output pulse signals provided from the pulse output section as drive signals by which the transistors are driven. On the other hand, when an absolute value of the actual current which flows in the transformer is larger than the target current, the holding circuit and the AND circuit stop the output of the drive signals. The drive circuit turns on and off the transistors on the basis of the drive signal provided from the holding circuit and the AND circuit.
The drive circuit is arranged for each of the transistors which are diagonally arranged in the switching circuit. The transistors arranged at opposite angle in the switching circuit are turned on and off at the same timing on the basis of the drive signal. That is, when a hard switching process of the switching circuit is executed, this process decreases an efficiency of the DC-DC converter because of increasing the switching loss.